Testing a fire detector sensor

ABSTRACT

A method is disclosed for testing the functionality of a sensor ( 1 ) of a fire detector during operation thereof. The method comprises applying a current-limited test signal to the sensor ( 1 ), the test signal being such that the impedance of the sensor is such as to absorb the current-limited test signal when the sensor is operating normally; and applying the output of the sensor to a test signal detector ( 7 ). The arrangement is such that the test signal passes the output terminal of the sensor ( 1 ) only when the sensor is not operating normally.

RELATED APPLICATION

This application claims the benefit of the prior foreign application GB0409759.8, filed Apr. 30, 2004. The entire teachings of the aboveapplication are incorporated herein by reference.

This invention relates to a method of testing a sensor of a firedetector, and to a fire detector which utilises that method. Theinvention is particularly concerned with the testing of anelectro-chemical sensor, but it is also applicable to any fire detectorsensor that has a low impedance between its monitored terminals.

There is a range of sensors used within fire detectors for theidentification of fires. In some markets, there is a requirement fortesting or monitoring each of the sensing components of fire detectorsfor integrity and correct operation.

It is desired that the operation of each sensor be electrically checkedby internal means to confirm that it is functioning correctly. This canbe done continuously in real time, or initiated on a regular basis byexternal control and indicating equipment. One type of sensor used toidentify a fire is an electro-chemical cell, an example of this being acarbon monoxide (CO) cell.

A method for checking the integrity of a CO cell in circuit is to applya voltage across the cell and evaluate its discharge characteristics.With this method, the CO cell is completely ineffective for many minutes(the CO monitoring system must be disabled to prevent a false alarm or afault indication) until it has been discharged to its nominal operatingvoltage. Also, additional circuitry is needed to perform this function,and this leads to an increase in size and complexity of the detector, aswell as an increase in the required power.

There are self-test systems (internal and external to such a sensor)that contain hydrogen or CO gas reservoirs/generators and gas releasemechanisms. However, these are usually intrusive (the CO monitoringsystem must be disabled to prevent a false alarm), draw a large amountof current, and are subject to environmental influences.

The present invention provides a method for testing the functionality ofa sensor of a fire detector during operation thereof, the methodcomprising the steps of:

-   a) applying a current-limited test signal to the sensor, the test    signal being such that the impedance of the sensor is such as to    absorb the current-limited test signal when the sensor is operating    normally; and-   b) applying the output of the sensor to a test signal detector;    wherein the arrangement is such that the test signal passes the    output terminal of the sensor only when the sensor is not operating    normally.

In a preferred embodiment, the test signal is supplied to the sensor bya pulse generator via a current limiter.

The sensor may be located on a detection module and the test signal maybe supplied to the detection module.

Advantageously, a remote DC signal is applied to the detection modulefor determining the year of manufacture of the sensor. Preferably, thetest signal and the DC signal are applied to the detection module on thesame electrical connection, wherein the DC signal may be monitored todetermine whether or not the electrical connection is made.

Preferably, the output of the sensor is applied to the detector via anamplifier.

The method may further comprise applying an offset voltage to theamplifier, so that the output of the amplifier is zero when the sensoris not operating normally.

Preferably, the test signal is such that the capacitance of the sensoris large enough to absorb the current-limited test signal when thesensor is operating normally.

The invention also provides a fire detector comprising a sensor fordetecting the presence of a fire, and a test circuit for testing thefunctionality of the sensor during operation thereof, the test circuitcomprising supply means for applying a current-limited test signal tothe sensor, and means for applying the output of the sensor to a testsignal detector, wherein the supply means is such that the impedance ofthe sensor is such as to absorb the current-limited test signal when thesensor is operating normally, and the arrangement is such that the testsignal passes the output terminal of the sensor only when the sensor isnot operating normally.

In a preferred embodiment, a pulse generator provides the test signal,and the test signal is supplied to the sensor via a current limiter.

Preferably, there is provided a detection module which comprises thesensor, a control module separate from the detection module whichcomprises the pulse generator, and an electrical connecting means toconnect the pulse generator to the detection module such that the testsignal is supplied to the sensor.

Preferably, the control module comprises a DC voltage supply meansarranged to supply the detection module with a DC voltage via theconnecting means. Advantageously, the control module comprises means forchecking the integrity of the electrical connection by monitoring the DCvoltage.

Advantageously, the detection module further comprises a resistivenetwork connected to the electrical connecting means, wherein theresistive value of the resistive network identifies the year ofmanufacture of the sensor. The control module may comprise a resistiveelement connected to the DC voltage supply means and a means formeasuring the current flowing through the said resistive element,wherein the resistive element may be arranged to form a resistor dividercircuit with the resistive network of the detection module such that themeans for measuring the current flowing through the resistive element isrepresentative of the year of manufacture of the sensor.

In a preferred embodiment, the current limiter is located on thedetection module.

Preferably, an amplifier is provided between the output terminal of thesensor and the detector. Advantageously, the amplifier is constituted byan op-amp and a feedback network.

The fire detector may further comprise means for applying an offsetvoltage to the amplifier, the arrangement being such that the output ofthe amplifier is zero when the sensor is not operating normally.Conveniently, a pedestal generator constitutes the means for applyingthe offset voltage to the amplifier.

Advantageously, a transistor is provided on the output side of thedetector and the amplifier, the transistor being effective to short outthe output of the amplifier when the test signal passes between theinput and output terminals of the sensor.

Preferably, the supply means is such that the capacitance of the sensoris large enough to absorb the current-limited test signal when thesensor is operating normally.

The invention also provides a fire detector system comprising a controlmodule having a means for generating and monitoring a DC signal, and adetection module having a sensor for detecting the presence of a fire,the control module and the detection module being electricallyconnected, the DC signal being applied to the electrical connectionbetween the control module and the detection module for testing theintegrity of the connection, wherein the control module provides awarning signal when the connection is not made.

Preferably, the detection module comprises a resistive network connectedto the DC signal, which resistive value determines the year ofmanufacture of the sensor, and which output is monitored by the controlmodule via the electrical connection.

The invention also provides a control module comprising a pulsegenerator for applying a test signal to a sensor, a DC supply voltagemeans, a resistive element connected to the DC supply voltage means,means for measuring the current flowing through the resistive elementand means for connecting the DC supply voltage to an external circuit,wherein the means for measuring the current signals an alarm when the DCsupply voltage is not connected to an external circuit.

The invention will now be described in greater detail, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a fire detector incorporating testmeans constructed in accordance with a first embodiment of theinvention;

FIG. 2 is a schematic diagram of a fire detector incorporating testmeans and means for determining the date of manufacture of a sensorconstructed in accordance with a second embodiment of the invention.

Referring to FIG. 1, a fire detector of the first embodiment comprises aCO cell 1, an amplifier circuit 2 constituted by an op-amp 2 a and afeedback network 2 b, and an output 3. The op-amp 2 a is configured forthe transimpedance mode, that is to say it converts the small currentgenerated by the CO cell 1 into a larger voltage via the feedbacknetwork 2 b, whilst maintaining zero voltage across the CO cell, therebyacting on the virtual earth principle. In use, the feedback network 2 bconverts the CO cell 1 current into a resultant voltage at the output 3.This network 2 b is usually a resistor, but it can be adjusted tocompensate for noise, EMC, tolerance and temperature characteristics.

The CO cell I is sensitive to minute concentrations of CO—a few partsper million (PPM). As CO is a gas usually produced in the very earlystages of a fire, the CO cell 1 is a very effective fire detectorsensor.

The drawing also shows elements of the test circuit of the invention,namely a test signal (pulse) generator 4 and a currentlimiting/decoupling network 5 upstream of the CO cell 1, a pedestalgenerator 6 feeding the + input of the op-amp 2 a, and a test signaldetector 7 and a transistor 8 at the output of the op-amp. The currentlimiting/decoupling network 5 reduces the current of the test signalgenerated by the pulse generator 4 to a level that will not affect thenormal operation of the CO cell 1 and the amplifier 2. Owing to thenature of the amplifier 2, the current of the test signal can be verylow, certainly much lower than that would affect the CO cell 1. Thenetwork 5 can also “decouple” the test signal, such that it will bereduced to a short pulse (as opposed to a continuous current) with theuse of a series capacitor. This will further eliminate the possibilityof the test signal affecting the CO cell 1 during normal operation.

In use, the pulse generator 4 provides a series of pulses to the CO cell1, these pulses being current limited by the network 5 to such an extentthat the capacitance of the CO cell is great enough to absorb thecurrent limited test signal, so that no resultant voltage will formacross the terminals of the CO cell. Under normal circumstances,therefore, the test signal will not be propagated through to the op-amp2 a, and so will remain undetected.

The CO cell amplifier circuit 2 must be capable of propagating the testsignal if the CO cell 1 has an open circuit fault. Therefore, if thetest signal has, for any reason, propagated past the CO cell terminals,been amplified by the op-amp 2 a and the feedback network 2 b, and isdetected by the test signal detector 7, it will initiate a fault signalto indicate a fault with the CO cell.

The fault can be indicated by the use of a separate signal, or (as shownin the drawing) by modification of the resultant CO amplifier output.For example, the amplifier circuit output can be set to give a‘pedestal’ output Vout, set by an offset voltage Vref generated by thepedestal generator 6 under normal conditions, but to give a zero outputto indicate a fault. Thus, if the CO cell 1 is removed from the circuit,an internal component within the cell is open circuit, the electrolytehas leaked away, or there is any other catastrophic fault, thecapacitance of the cell will not be present, and the test signal willpass through the cell to be amplified by the amplifier circuit 2.Consequently, the test signal will be detected by the test signaldetector 7 if the capacitance of the CO cell 1 is not present for anyreason. If so, the output of the detector 7 will turn the transistor 8(which may be a bipolar transistor or a FET) on. This in turn will shortout the output of the op-amp 2 a, hence removing the pedestal from theresultant output voltage Vout.

Vout is a function of the test circuit. If there is no fault in the COcell 1, Vout will be proportional to the concentration of CO plus thepedestal voltage, that is to say Vout=Vref+α, where α is a parameterthat is proportional to the CO concentration. If there is a fault in theCO cell 1, Vout=0 volt. For example, if Vref is 1 volt, and the gain ofthe amplifier gives 0.1 volt per PPM of CO, a Vout of 1 volt means thatthe CO level is 0 PPM. Similarly, a Vout of 2 volts means that the COlevel is 10 PPM. As it is impossible to have a negative PPM of CO, theVout will only fall below 1 volt (the pedestal voltage) if there is afault with the CO cell 1. This approach is advantageous if there is alimitation on the number of channels available to report the status ofthe CO concentration and the test circuit.

FIG. 2 shows the second embodiment. The second embodiment is similar tothe first, and only the differences will be described. Like referencenumerals are used for like parts.

The fire detector of the second embodiment comprises a detection module10 electrically connected to a control module 11 via two connectinglines HVC and 0V.

The detection module 10 includes the current limiting/decoupling network5, the pedestal generator 6, the CO cell 1, the amplifier circuit 2, thetest signal detector 7 and the transistor 8. The detection module alsoincludes a resistive network 11 connected between the connecting linesHVC and 0V, the resistive network 11 being AC coupled to the currentlimiting/decoupling network 5 via a capacitor (not shown). The values ofthe resistors comprising the resistive network 11 are chosen to identifythe year of manufacture of the CO cell 1.

The control module 11 includes the test signal pulse generator 4, a DCvoltage supply 12 and a current measuring circuit 13. The DC voltagesupply 12 is connected to the resistive network 11 via the HVCconnecting line and two series resistors (not shown), one of which islocated at the output of the control module 11, the other of which islocated at the input of the detection module 10. The current monitoringcircuit 13 comprises a resistive element (not shown) of a fixed valuewhich, in combination with the resistive network 11, forms a resistordivider network.

In use, the pulse generator 4 provides a series of test pulses to the COcell 1 via the connecting lines HVC and 0V and the currentlimiting/decoupling network 5. The CO cell 1 is tested as described inthe first embodiment, the only difference being that the pulse generator4 is located on the control module 11 which is remote from the detectionmodule 10 containing the CO cell 1.

The DC voltage supply 12 generates a DC voltage which, when the controlmodule 11 is connected to the detection module 10 via the HVC connectionline, develops across the total resistor divider network including theresistive network 11. The DC voltage is prevented from affecting theoperation of the remainder of the detection module 10 because thecurrent limiting/decoupling network 5 is AC coupled to the resistivenetwork 11. The current flowing through the resistor of the currentmeasuring circuit 13 for any given DC supply voltage is thereforedetermined by the values of the resistors in the resistive network 11,which have been chosen to identify the year of manufacture of the COcell 1. By measuring the current in this way, the year of manufacture ofthe CO cell may be determined. In this embodiment, the values of theresistive network 11 are chosen such that the measured current is inproportion to the date of manufacture, for example:

-   2006=100 mV-   2007=200 mV-   2008=300 mV-   2009=400 mV    etc.

The date information is then relayed to control and indication equipment(not shown). This allows a user to identify detection modules 10 wherethe CO cell 1 has exceeded its guaranteed operating lifetime, thusprompting servicing action.

The integrity of the HVC line can be determined by regularly checkingthat the DC voltage or current in the control module 11 is not at anunusual level. This test is useful as it indicates whether or not thetest pulses are being successfully transmitted to the detection module10. Without this check, if the HVC line is not connected properly, thetest pulses would not be transmitted to the CO cell 1 and no faultcondition would be detected if the CO cell were open-circuited orremoved.

It will be apparent that the test circuit described above could bemodified. For example, the test signal detector 7 could be set tomonitor for a voltage level below Vref, or for abnormally fast edges.Moreover, extra circuitry could be added to synchronise the test signaldetector 7 to the pedestal generator 6, such that it will inhibit thefault signal to minimise the reporting of a false result.

Although the pedestal generator 6 constitutes an integral part of thetest circuit, the configuration of the power supplies for the op-amp 2 amay require the presence of the pedestal generator even if testing ofthe CO cell 1 is not required. For example, the Vref output by thepedestal generator 6 could be used to stop the output of the op-amp 2 asaturating near zero volts. Where the test circuit is incorporated, thefault signal is generated directly from the test signal detector 7.

It is also possible to use other forms of test signal. Thus, the testsignal can be derived from any source, for example from the system clockor by using a timing pulse from an unrelated function. Moreover, thetest signal generator 4 can be realised by a pull-up or a pull-downconfiguration, for example by an open collector constant current sink.Furthermore, as indicated above, the fault signal can be indicated bythe use of a separate signal which can be fed into, for example, amicroprocessor or a transducer.

Finally, although the test circuit described above is used with a COcell 1, it will be apparent that it could be used for monitoring otherelectrochemical cells which have a low impedance, or indeed any otherfire detector sensor that has a low impedance between its monitorterminals.

It will be apparent that the test circuit described above has a numberof advantages. In particular, testing can be carried out while the COcell 1 is in circuit, so that the cell does not need to be removed ordisabled for testing to be carried out. Thus, the CO cell 1 and itsassociated circuits will continue to operate normally while testing iscarried out. Moreover, no long term potential is applied to the CO cell1, thereby avoiding the cell having a recovery time in which it is notusable.

The main advantage of the test circuit described above is, therefore,that it is able to indicate a fault when there is an error relating tothe operation of the CO cell 1. Without the test circuit of theinvention, when there is no stimulating gas present in the cell, itsnature means that it will not generate or leak any voltage or current.The characteristics of the cell will, therefore, not be any different ifthere is a fault, or if the cell is not even fitted. The provision ofthe test circuit thus provides an indication of the integrity of the COcell 1 within the fire detector circuit.

Another advantage of the test circuit described above is that it isnon-intrusive, so it does not require the CO cell monitoring system tobe disabled while a test is carried out. The test process will,therefore, not alter the effectiveness of the CO cell 1 (or itsassociated circuitry) at any time whilst measuring levels of COconcentration. Moreover, the control and indicating equipment associatedwith the detector can receive real time data regarding the integrity ofthe CO cell 1.

Another advantage of the test circuit described above is that it willnot result in significant degradation of the performance of the CO cell1 over its lifetime. Consequently, testing can be applied continuously,without problems arising relating to worn out or damaged components.This means that the associated control and indicating equipment canreceive continuous feedback about the integrity of the CO cell 1,without affecting its performance.

Another advantage of the test circuit described above is that it doesnot require the use of a test gas or other stimuli to confirm theoperation of the CO cell 1. This means that the test can be appliedcontinuously, without problems arising relating to exhausted components.

1. A method for testing the functionality of a sensor of a fire detectorduring operation thereof, the method comprising the steps of: a)applying a current-limited test signal to the sensor, the sensor havingan impedance such that sensor absorbs the current-limited test signalwhen the sensor is operating normally; and b) applying the output of thesensor to a test signal detector via an amplifier circuit; wherein thearrangement is such that, during testing, the output voltage of theamplifier circuit is substantially constant when the sensor is operatingnormally, and such that the test signal passes the output terminal ofthe sensor only when the sensor is not operating normally, whereby thetest signal is amplified by the amplifier circuit thereby to provide asignal indicative of a sensor fault.
 2. The method as claimed in claim1, wherein the test signal is supplied to the sensor by a pulsegenerator via a current limiter.
 3. The method as claimed in claim 1,wherein the sensor is located on a detection module and the test signalis supplied to the detection module.
 4. The method as claimed in claim3, further comprising applying a remote DC signal to the detectionmodule for determining the year of manufacture of the sensor.
 5. Themethod as claimed in claim 4, wherein the test signal and the DC signalare applied to the detection module on the same electrical connection,and wherein the DC signal is monitored to determine whether or not theelectrical connection is made.
 6. The method as claimed in claim 1,further comprising applying an offset voltage to the amplifier circuit,so that the amplifier circuit is zero when the sensor is not operatingnormally.
 7. The method as claimed in claim 1, wherein the test signalis such that capacitance of the sensor is large enough to absorb thecurrent-limited test signal when the sensor is operating normally. 8.The fire detector comprising a sensor for detecting the presence of afire, and a test circuit for testing the functionality of the sensorduring operation thereof, the test circuit comprising supply means forapplying a current-limited test signal to the sensor, and means forapplying the output of the sensor to a test signal detector, via anamplifier circuit, wherein the sensor has an impedance such that thesensor absorbs the current-limited test signal when the sensor isoperating normally, and the arrangement is such that the output voltageof the amplifier circuit is substantially constant when the sensor isoperating normally, and such that the test signal passes the outputterminal of the sensor only when the sensor is not operating normally,whereby the test signal is amplified by the amplifier circuit thereby toprovide a signal indicative of a sensor fault.
 9. The fire detector asclaimed in claim 8, wherein a pulse generator provides the test signal,and the test signal is supplied to the sensor via a current limiter. 10.The fire detector as claimed in claim 9, wherein there is provided adetection module which comprises the sensor, a control module separatefrom the detection module which comprises the pulse generator, and anelectrical connecting means to connect the pulse generator to thedetection module such that the test signal is supplied to the sensor.11. The fire detector as claimed in claim 10, wherein the control modulecomprises a DC voltage supply means arranged to supply the detectionmodule with a DC voltage via the connecting means.
 12. The fire detectoras claimed in claim 11, wherein the control module comprises means forchecking the integrity of the electrical connection by monitoring the DCvoltage.
 13. The fire detector as claimed in claim 11, wherein thedetection module further comprises a resistive network connected to theelectrical connecting means, and wherein the resistive value of theresistive network identifies the year of manufacture of the sensor. 14.The fire detector as claimed in claim 13, wherein the control modulecomprises a resistive element connected to the DC voltage supply meansand a means for measuring the current flowing through the said resistiveelement, and wherein the resistive element is arranged to form aresistor divider circuit with the resistive network of the detectionmodule such that the means for measuring the current flowing through theresistive element is representative of the year of manufacture of thesensor.
 15. The fire detector as claimed in claim 10, wherein thecurrent limiter is located on the detection module.
 16. The firedetector as claimed in claim 8, wherein the amplifier circuit isconstituted by an op-amp and a feedback network.
 17. A fire detector asclaimed in claim 8, further comprising means for applying an offsetvoltage to the amplifier circuit, the arrangement being such that theoutput of the amplifier circuit is zero when the sensor is not operatingnormally.
 18. The fire detector as claimed in claim 17, wherein apedestal generator constitutes the means for applying the offset voltageto the amplifier circuit.
 19. The fire detector as claimed in claim 8,wherein a transistor is provided on the output side of the detector andthe amplifier circuit, the transistor being effective to short out theoutput of the amplifier circuit when the test signal passes the outputterminal of the sensor.
 20. The fire detector as claimed in claim 8,wherein the supply means is such that capacitance of the sensor is largeenough to absorb the current-limited test signal when the sensor isoperating normally.
 21. A fire detector system comprising a controlmodule having a means for generating and monitoring a DC signal, and adetection module having a sensor for detecting the presence of a fire,the control module and the detection module being electricallyconnected, the DC signal being applied to the electrical connectionbetween the control module and the detection module for testing theintegrity of the connection, wherein the control module provides awarning signal when the connection is not made.
 22. The fire detectionsystem as claimed in claim 21, wherein the detection module comprises aresistive network connected to the DC signal, which resistive valuedetermines the year of manufacture of the sensor, and which output ismonitored by the control module via the electrical connection.
 23. Acontrol module comprising a pulse generator for applying a test signalto a sensor, a DC supply voltage means, a resistive element connected tothe DC supply voltage means, means for measuring the current flowingthrough the resistive element and means for connecting the DC supplyvoltage to an external circuit, wherein the means for measuring thecurrent signals an alarm when the DC supply voltage is not connected toan external circuit.